In anticipation of the release date, we've published official modding tutorials for Project Highrise. Once you get the game, you will be able to make your own decorations, furniture, offices, restaurants, and more!
See this post for details: http://steamcommunity.com/app/423580/discussions/0/343785574528211729/
Also please check out the workshop forum to talk with other modders and share your creations:
http://steamcommunity.com/workshop/discussions/?appid=423580
Happy modding!